1. Field of the Invention
The present invention relates to a subscriber premises side optical network unit and an optical transmission system and more specifically to an optical network unit installed at a subscriber premises side terminal of an optical transmission path and the optical transmission system having the same.
2. Description of Related Art
A PON (Passive Optical Network) type optical transmission system that branches an optical transmission path connected to a center by a passive type splitter to lay the paths to a plurality of subscriber premises is used in an optical network such as FTTH and CATV as described in Japanese Patent Application Laid-open No. Hei. 9-214541 for example. This sort of optical transmission system is called also as PDS (Passive Double Star).
As shown in FIG. 9, a splitter 903 is connected to an optical fiber 902 led from an OLT (Optical Line Termination) 901 of a center to the side of users and subscriber premises ONUs (Optical Network Unit) 905-1 through 905-n are connected to a plurality of optical transmission paths branched by the splitter 903 through optical fibers 904-1 through 904-n in the PON-type optical transmission system.
FIG. 10 shows a structure of the ONU 905-1 as one example of the ONUs 905-1 through 905-n. The ONU 905-1 includes an optical transceiver section 906, an ONU functioning section 907 and Ethernet (registered mark) interface 908 and is installed in premises of each subscriber. The optical transceiver section 906 is connected with the optical fiber 904-1 led into the premises from the outdoor for example through an optical termination unit 909.
As the Ethernet (registered trademark) interface 908, 10/100/1000 BASE-TX and the like may be used and a LAN cable 910 for example may be connected to the interface. Terminal units 914 such as a computer and a printer may be directly connected to the LAN cable 910. Or, a switch such as a router 912 and a switching hub 913 may be connected to the LAN cable 910 and other terminal units 914 may be connected to the switch.
In order to enable the terminal units, switch and others (referred to as external nodes hereinafter) connected to the ONU to communicate with the center through the ONU, a process for enabling the communication between the center side OLT and the subscriber premises side ONU is made at first and then a process for starting the communication between the ONU and the external node is made. An auto-negotiation function is provided respectively in a MAC (media Access Control) section within the ONU and in a MAC section of the external node in order to carry out the process for starting the communication between the ONU and the external node.
However, when some kind of signal, e.g., a signal for carrying out the auto-negotiation, is outputted out of the ONU to the external node before the communication is enabled between the center side OLT and the subscriber premises ONU, the external node starts a predetermined process by receiving the signal from the ONU and outputs data related to the auto-negotiation for example to the ONU. As a result, there is a possibility that the ONU executes an error handling process, thus disabling to start the communication.
Still more, it is necessary to quickly halt transmission of signals between the ONU and the external node when the communication between the OLT and the ONU is disallowed by some reason after when the communication between the external node and the center through the ONU is enabled. If the ONU outputs signals to the external node even after when the communication between the OLT and the ONU is disabled, the external node determines that the communication is enabled and continuously and normally outputs signals to the ONU. As a result, there is also a possibility that the ONU executes the error handling process, thus disabling to restart the communication.
Accordingly, the invention is made to solve these problems and seeks to provide a subscriber premises side optical network unit, and an optical transmission system having the same, that monitors whether or not a communication condition with the center side optical line termination is established and halts signals to be outputted to the external node when the communication condition is not established.